Cobalt base alloys



United States Patent 3,383,205 COBALT BASE ALLOYS Chester T. Sims,Ballston Lake, and Allan D. Foster, Schenectady, N.Y., assignors toGeneral Electric Company, a corporation of New York No Drawing. FiledDec. 14, 1964, Ser. No. 418,263 3 Claims. (Cl. 75-171) ABSTRACT OF THEDISCLOSURE High temperature alloys which are resistant to oxidizing andcorrosive influence consist in percent by weight essentially of carbon0.1 to 0.60, chromium 27.0 to 35.0, nickel 9.3 to 11.5, tungsten 6.0 to8.0, iron 6.0 maximum, and boron in an effective amount up to 0.050maximum to impart ductility with the remainder essentially cobalt.

This invention relates to new and useful alloys. More particularly, itrelates to alloys which are capable of operaung under elevatedtemperature conditions and which are resistant to oxidizing,sulfidizing, and other corrosive combustion gases at such temperatures.

It is Well known that equipment depending upon the driving force ofcombustion gas, such as gas turbines, operate more efiiciently and withgreater power output at elevated temperatures. It is also Well knownthat at such elevated temperatures the physical or mechanical strengthof many materials often decreases drastically and that they tend tobecome subject to excessive oxidation and/ or corrosion caused byoxidizing atmospheres, often complexed with other contaminants whichenter from the fuels or atmosphere, including sulfur, vanadium, sodiumand others. As the operating temperature of such equipment rises,relatively small improvements in the oxidation and corrosion resistanceand strength of such materials become more and more important. Forexample, in gas turbines operating at temperatures of the order of about1600 F., an improvement of only one hundred degrees Fahrenheit in theresistance of the materials of construction represents a notableadvance. For example, in a typical gas turbine, an increase in operatingtemperature from about 1500 F. to about 1600 F. represents an increasein power output of about 14% and an increase in efiiciency of about 1 to5%.

The use of cobalt-base alloys containing relatively large amounts ofchromium for high temperature operation under oxidative and corrosiveconditions is well known. However, it has been the teaching of the priorart that increases in the chromium content of such alloys over about 25%by weight actually result in an increase in scaling or deterioration.This teaching is set forth, for example, in Journal of theElectrochemical Society, vol. 103, No. 8, Phalnikar et al., entitledHigh Temperature Scaling of Cobalt-Chromium Alloys. Thus, according tothe prior art teaching, increases in chromium over 25% by weight wouldresult in less suitable elevated temperature operation in terms ofoxidation or corrosion resistance, or both.

It is a primary purpose of the present invention to provide new anduseful alloys of the cobalt-chromium class which are characterized bytheir ability to operate at increasingly elevated temperatures undermore oxidative and corrosive conditions than heretofore, such as in gasturbines. Another object is to provide such alloys which are useful inother equipment and devices subjected to elevated temperatures andoxidizing or corrosive atmospheres, or both, such as furnaces and thelike.

Briefly, there are provided by the present invention high-temperature,corrosion-resistance cobalt-base alloys having a percent by weightcontent of carbon 0.1 to 0.60, chromium 27.0 to 35.0, nickel 9.3 to11.5, tungsten 6.0 to 8.0, iron 6.0 maximum, boron 0.050 maximum,manganese 1.0 maximum as an impurity not to be added, with the remaindercobalt except for residual impurities such as phosphorus, sulfur,silicon and the like. It has been found that alloys having balancedmetallic compositions within the limits specified are characterized bysubstantial increases in corrosion and oxidation resistance, while atthe same time having a suitably high rupture strength and ductility forsuch high temperature operation. The materials are also particularlyuseful in that they are readily weldable, permitting the fabrication ofvarious shaped structures.

The teaching of the present invention proceeds contrary to the prior artteaching as set forth above, i.e., that in cobalt-chromium hightemperature alloys, optimum oxidation and corrosion resistance isobtained at about 25% by weight of chromium. However, the presentcompositions represent carefully balanced combinations of constituents,each of which contributes to the desirable characteristics obtained.Deviations in the proportions of the materials destroy this balance, andresult in materials which have been found to be wanting in one or moreessential characteristics. For example, when the carbon content islowered beyond that prescribed, there results an undesirable loss ofstrength. On the other hand, increasing the carbon content above thatstated detracts severely from the weldability of the material, as wellas the rupture ductility, and also lowers the oxidation resistance.Reduction of the chromium content below that set forth results in adetrimental loss of oxidation resistance, whereas increases above theprescribed range of chromium result in a loss in ductility, and areaccompanied only by a very marginal increase in oxidation or corrosionresistance, which is more than offset by the loss in ductility. Thenickel, tungsten and iron contents do not appear to be particularlycritical for oxidation and corrosion resistance, but it has been founddesirable to hold them within the stated ranges for mechanical andphysical property reasons. Boron imparts ductility to the alloy whenadded, but increasing the boron content beyond that set forth causesdetrimental low-melting phases to form in the alloy. The cobalt base, ofcourse, is well known for its contribution to sulfidation and oxidationresistance. Various impurities are present in the alloys. Manganese canbe tolerated in amounts up to about 1.00%, but is not purposely added inany case. Other impurities such as phosphorous and sulfur are held to amaximum of about 0.04% each.

The following examples will illustrate the practice of the invention; itbeing realized that they are examplary only and not to be taken aslimiting in any way.

EXAMPLE 1 There was prepared an .alloy consisting of, by weight percentdetermined from final chemical analysis, carbon 0.17, chromium 31.8,nickel 9.7, tungsten 7.69, iron 0.99, boron 0.0073, manganese 0.54 withabout 0.01 of phosphorous and sulfur as impurities with the remainderessentially cobalt. This alloy was cast into pieces having dimensions of1% inches by 5 inches from which test pieces one inch in diameter and0.060 inch thick were cut for testing, and compared with a prior artalloy having a percent by weight content of carbon 0.23, chromium 25.5,nickel 10.5, tungsten 6.9, iron 0.82, boron about 0.007 with manganese0.54, and about 0.02 each of phosphorous and sulfur as impurities andwith the remainder essentially cobalt. It will be noted that the presentalloy has a higher amount of chromium than the prior art alloy which inits larger present .amounts has been found to impart unexpected andadvantageous qualities.

When test pieces of the above example and the above prior art materialwere evaluated by being placed edgewise to the gas stream direction in asmall burner apparatus (which simulates actual gas turbine operatingconditions) at a temperature of 2000 F., burning natural gas with an airto fuel Weight ratio of 50 to 1, the oxidative corrosion in mils perside of the prior art material after 600 hours was 14.6. On the otherhand, .the material of the above example had corresponding corrosionafter 594 hours of only 7.3 mils per side. After 984 hours, the priorart material exhibited corrosion of 20.4 mils per side, but even after2553 hours the present material exhibited corrosion of only 14.3 milsper side. It will thus be seen that the present alloy provides a decidedadvantage over the prior art material insofar as oxidation resistance atelevated temperatures is concerned.

The material of the above example was also tested under sulfidizingconditions with the above prior art ma terial in a test burner operatedat a temperature of 1600 F. with an airto-fuel weight ratio of 68 to 1,burning .a distillate oil containing 3.8% by weight of sulfur, to whichthere had been added 325 parts per million of sodium chloride. Thisproduces an extremely corrosive environment. In this test, deposition ofmolten material can take place on the leading edge of the test specimen,causing a higher penetration rate than where the deposition does notoccur. The data presented here are both the minimum attack (no moltendeposition) and the maximum attack where molten material resided. After465 hours under such conditions, a typical prior art material had acorrosion in mils per side of the test specimen ranging from about 3.8(minimum) to 18.1 (maximum) mils. On the other hand, the presentmaterial, even after 570 hours of operation under such rigorousconditions, had only about 1.0 mil per side of corrosion overall.

As pointed out above, the alloys of the present invention are not onlycharacterized by good oxidation and sulfidation resistance, but theirfabrication into various shaped structures is facilitated by the factthat they are readily weldable.

The rupture strength of the material of the above example after hours ata temperature of 1600 F. is about 9,500 p.s.i., which compares veryfavorably with the corresponding value of 10,500 p.s.i. for the aboveprior art material. The rupture ductility of the above example materialis equivalent to that of the prior art material.

EXAMPLE 2 An alloy was prepared consisting of, by weight percent, carbon0.19, chromium 27.6, nickel 9.3, tungsten 7.5, iron 5.6, boron 0.0069,manganese 0.56, with the remainder essentially cobalt. The alloy soprepared was cast as in Example 1, and similar test pieces prepared.

When such test pieces were tested for oxidative corrosion in natural gasas in Example 1, the corrosion in mils per side after 594 hours was 7.4mils. After 2431 hours under the above oxidative conditions, thecorrosion per side was 18.4 mils. It will be seen that the material ofExample 2 provides a decided advantage over the prior art materialmentioned above. The rupture strength of the material of this example at10 hours at a temperature of 1600 F. is about 10,500 p.s.i., which isthe same as the corresponding value for the above prior art material.The rupture ductility is also equivalent to that of the prior artmaterial, and the weldability was found to be good.

There are provided, then, by the present invention, new and usefulalloys which are particularly characterized by their high strength atelevated temperatures and their ability to withstand at such elevatedtemperatures corrosive attack caused by oxidizing and sulfidizingcombustion gas constituents. They are particularly useful forfabricating gas turbine components such as nozzle partitions, as well asother static or moving parts of such equipment which are exposed to hightemperature corrosive conditions. They are also useful in general infurnaces and other equipment which operate under similar rigorousconditions, where they can serve as materials of construction for fans,runners, and the like.

What We claim as new and desire to secure by Letters Patent of theUnited States is:

1. A high temperature resistant alloy, consisting essentially of byweight carbon 0.1 to 0.60 percent, chromium 27.0 to 35.0 percent,tungsten 6.0 to 8.0 percent, nickel 9.3 to 11.5 percent, boron in aneffective amount up to 0.050 percent maximum to impart ductility, iron6.0 percent maximum, with the remainder essentially cobalt.

2. A high temperature resistant alloy consisting essentially of byweight carbon 0.17 percent, chromium 31.8 percent, tungsten 7.69percent, nickel 9.7 percent, boron 0.0073 percent, iron 0.99 percent,with the remainder essentially cobalt.

3. A high temperature resistant alloy consisting essentially of byweight carbon 0.19 percent, chromium 27.6 percent, tungsten 7.5 percent,nickel 9.3 percent, boron 0.0069 percent, iron 5.6 percent, with theremainder essentially cobalt.

References Cited UNITED STATES PATENTS 2,744,010 5/ 1956 Calloway -1712,746,860 5/1956 Binder et al. 75171 2,855,295 10/1958 Hansel 751712,996,379 8/1961 Faulkner 75-171 I-IYLAND BIZOT, Primary Examiner.

DAVID L. RECK, RICHARD O. DEAN, Examiners.

